Electrophotography combines the photoconductivity of materials with electrostatic phenomena to form an image, as disclosed in U.S. Pat. No. 2,297,691 to Carlson. Electrophotographic images are generally formed by the following steps: (i) uniformly charging the surface of a photoconductor in the dark by corona-discharge, (ii) forming an electrostatically latent image by exposing the charged surface of the photoconductor to the image, and (iii) depositing colored charge-carrier particles (e.g., toner) onto the electrostatic latent image to convert the latent image into a visible image of the toner. The visible image of the toner is thereafter transferred onto a support such as paper. Electrophotography has been employed widely in copying machines, laser beam printers, and other types of output devices of computers, instruments, and other similar apparatuses.
The photoconductor includes a conductive substrate and a photoconductive film placed onto the conductive substrate. Inorganic photoconductive materials such as amorphous silicon, selenium, zinc oxide and cadmium sulfide have been conventionally used as the main component of the photoconductive film. Unfortunately, however, these traditionally used materials have certain disadvantages. Cadmium sulfide, for example, has been identified to be a potential health hazard, as well as a potential environmental pollutant. In addition, the manufacturing costs associated with the use of silicon and/or selenium (e.g., amorphous silicon, amorphous selenium and/or selenium alloy) are too high due to the required use of the vacuum deposition method or the CVD method in forming such photoconductive films. Furthermore, photoconductive films containing amorphous silicon, amorphous selenium or selenium alloy exhibit poor flexibility.
To obviate the foregoing problems, efforts have been initiated towards investigating the use of organic photoconductors which have a photoconductive film containing an organic photoconductive material dispersed and dissolved into a resin binder. These organic materials are advantageous because (i) there is a wide variety of useful materials, (ii) film formation is easier, (iii) manufacturing costs are decreased, and (iv) the resulting photoconductors are thermally stable. As a result of these advantages, organic photoconductors are predominantly used today.
Photoconductors operate by first generating electric charges in response to received light and then transporting the generated electric charges. Two types of photoconductors which are effective in these functions are mono-layered photoconductors and function-separated photoconductors. Mono-layered photoconductors have a single photoconductive film which exhibits all the above described functions. Function-separated photoconductors have a photoconductive film laminate comprising at least (i) a charge generation layer which generates charge carriers in response to the received light and (ii) a charge transport layer which retains surface potential of the photoconductor in the dark and which neutralizes the surface potential by transporting the charge carriers, which are generated in the charge generation layer. Function-separated photoconductors have been predominantly used recently because appropriate materials can be easily selected for each of the charge generation layer and the charge transport layer. This advantage helps to obtain greater photoconductive sensitivity and, as a result, excellent electrophotographic properties.
Since charge carrier mobility in the charge generation layer is essentially low, it is necessary for the charge generation layer to be substantially 1 .mu.m or less in thickness. To protect such a thin charge generation layer against wear, practical function-separated photoconductors include a laminate-type photoconductive film in which the charge transport layer is laminated onto the charge generation layer.
At present, the most prevalent type of function-separated photoconductors are negative charging photoconductors because the vast majority of practical charge transport agents available at the present time can only transport holes (e.g., a charge in a substance which is the same amount of and opposite to the charge of an electron). Unfortunately, useful charge transport agent which can transport electrons has not yet been found. Negative-charging-type photoconductors work by forming the image by charging the surface of the photoconductor with a negative electrostatic charge. Typically, corona discharged is used to produce the electrostatic charge.
However, there are several disadvantages with the use of negative corona discharge. First, negative corona discharges are not uniform. As a result, the surface of the photoconductor becomes unevenly charged, and the images obtained therefrom lack smoothness and definition. Secondly, negative corona discharge causes a large amount of ozone (e.g., ten times as much as by the positive corona discharge). Since ozone is thought to cause deterioration of photoconductor surface materials and their resulting electrical properties, a large amount of ozone shortens the life of the photoconductor and decreases the quality of the output images. In addition, a large amount of ozone is thought to be hazardous to the environment.
To obviate the foregoing problems, research has recently been redirected to efforts in finding and/or developing organic photoconductors which can be used in the positive charging mode. Positive-charging-type organic photoconductors may be classified as a mono-layered type (e.g., one which includes a photoconductive film for charge generation and for charge transport) or a function-separated type (e.g., one which includes a photoconductive film including a layer for charge generation and another layer for charge transport).
For example, Japanese Unexamined Laid Open Patent Application No. S48-25658 discloses a mono-layered-type photoconductor which uses poly-N-vinylcarbazole chemically sensitized with an acceptor material such as 2,4,7-trinitrofluorenone (TNF) and tetracyanoquinodimethane (TCNQ). In addition, Japanese Unexamined Laid Open Patent Application No. S47-10785 discloses a mono-layered-type photoconductor which uses an eutectic complex consisting of a pyrylium salt dye and a resin.
However, when an acceptor material such as TNF and TCNQ for sensitizing poly-N-vinylcarbazole is added in sufficient quantities to obtain practical sensitivity, the dark resistance of the photoconductor is decreased by the formation of a charge transfer complex, thereby decreasing the charge level of the photoconductor. Moreover, TNF and TCNQ are thought to be too carcinogenic to be used in general-purpose photoconductors. Similarly, photoconductors containing ionic dye compounds (such as a pyrylium salt or a thiapyrylium salt) are unacceptably sensitive to variations in humidity (e.g., high humidity lowers the level of charge). Therefore, charge transfer complex forming materials and ionic materials are not practical for use as photoconductive materials.
Japanese Examined Patent Application No. S47-42512 discloses a mono-layered-type photoconductor which uses X-type metal-free phthalocyanine, an n-type semiconductor (e.g., containing a semiconducting substance wherein electrons, not holes, are the dominant charge carriers), dispersed in a binder resin. X-type metal-free phthaloxyanine materials are also disclosed in U.S. Pat. No. 3,357,989, which reference is incorporated herein by its entirety. However, this photoconductor poses some problems in its sensitivity and characteristics in repeated use due to its insufficient charge transport capability.
Although pigment-dispersion type photoconductors which use both a hole transport agent and an electron transport agent have been tested to determine whether the charge transport capability can be improved, few useful electron transport agents have been identified for use in photoconductors. Many potential electron transport agents are toxic or carcinogenic. In addition, a large number of electron transport agents used in photoconductive films cause injection of electrons (e.g., having the opposite charge) from the negatively charged substrate. As a result, these electron transport agents lower the resistance of the photoconductive film because charge transfer complexes are formed with the charge generation agent or with the hole transport agent. Inevitably, the charging capability of the photoconductor is impaired.
During use, the photoconductor surface is eroded by repetitive sliding-contact with the toner, carrier paper, and cleaning blade. As the photoconductive film becomes thinner, the problems associated with repeated printing become more prevalent (e.g., a decrease in the potential retention capability and/or a decrease in printing density of the output images). Although a thicker photoconductive film is more durable, the benefits of durability must be balanced with the benefits of photoconductive sensitivity which is adversely affected by an increase in photoconductive film thickness beyond that at which the maximum sensitivity is obtained.
Recently, the function-separated-laminate-type photoconductor has been investigated intensively, because a photoconductor having a photoconductive film, which includes a charge transport layer laminated onto a charge generation layer, can be used in the positive charging mode if a charge transport agent which exhibits excellent electron transport capability is found. However, an electron transport agent having such properties has not yet been found. To obtain a function-separated-laminate-type photoconductor for use in the positive charging mode in light of practical charge transport agents presently available, it is necessary to employ a photoconductive film which includes a charge generation layer laminated onto a charge transport layer exhibiting a hole transport capability. This type of photoconductive film, which has an order of lamination opposite to that in the photoconductor used in the negative charging mode, is called an "inverse-lamination-type photoconductor". As described earlier, the charge generation layer of the function-separated-laminate-type photoconductor should be as thin as 1 .mu.m or less. It is difficult to form such a thin, uniform film. Such a thin film is easily adversely affected by damage, unevenness, stains and deposits such as contaminants on the under layer. Such film defects and uneven film thickness further cause image defects such as uneven printing density, black spots and white streaks. Film defects further decrease productivity and raise manufacturing costs of the photoconductor. As a result, inverse-lamination-type photoconductors, which include such a thin charge generation layer in its surface, exhibit insufficient durability against repeated printing.
Unfortunately, a wear-resistant protection film, which can protect the photoconductor surface from wear, unacceptably increases manufacturing costs by adding an additional manufacturing step. Furthermore, such a protective film adversely affects the electrical properties of the photoconductor because it does not exhibit any charge transport capability. Therefore, a photoconductor having a surface protection film has not yet been used in practice.
Japanese Examined Patent Application No. H05-30262 discloses a photoconductor which obviates the foregoing problems of the mono-layered-dispersion-type photoconductor and the inverse-lamination-function-separated type photoconductor. This photoconductor includes a photoconductive film which further includes a charge transport layer, which contains a hole transport agent, laminated onto a conductive substrate, and a layer, which contains both a charge generation agent and a hole transport agent, laminated onto the charge transport layer. Hereinafter, the layer which exhibits both charge generation and charge transport functions will be referred to as a "charge generation and transport layer". Similarly hereinafter, a photoconductor which includes a charge generation and transport layer laminated on a charge transport layer will be referred to as a "mono-layer-dispersion-type inverse-laminate photoconductor". The mono-layer-dispersion-type inverse-laminate photoconductor improves durability against repeated printing as compared with the inverse-laminate-function-separated-type photoconductor, the surface layer of which only exhibits charge generation function. An increase in durability is seen because the surface layer of the mono-layer-dispersion-type inverse-laminate photoconductor may be set to be thicker than the surface layer of the inverse-laminate-function-separated-type photoconductor. However, when the charge generation and transport layer is too thick, the electrostatic capacity of the photoconductive film lowers and the amount of electrical charges retained in the photoconductive film decreases, even if the potential retention capability is improved. As the amount of electrical charges retained in the photoconductive film decreases, it becomes difficult to attract the toner thereby causing electrical fatigue such as lowered printing density in the output images, lowered sensitivity in repeated use, a rise in residual potential, and variation of charging capability. During use, photoconductor surface wear by the toner and the carrier paper sometimes cause local scratch streaks ten times as deep as the average scratch streak depth. Since electrical charges can not escape from such deep scratch streaks as to reach the charge transport layer, defects like scratches are caused on the output images. Thus, the conventional mono-layer-dispersion-type inverse-laminate photoconductor still fails to be sufficiently durable against repeated printing.
As explained hereinbefore, the conventional positive-charging type photoconductor is impractical as compared to the negative-charging type photoconductor.
In view of the foregoing, it is an object of the invention to provide a positive-charging type photoconductor which exhibits excellent electrical properties and excellent durability during repeated printing.
It is another object of the invention to provide a positive-charging type photoconductor, the characteristics of which are relatively unaffected by the changes of the circumstances and by the repeated use thereof.
It is still another object of the invention to provide a stable positive-charging type photoconductor which facilitates obtaining high-quality output images.
These and other objects of the present invention will become apparent in light of the following disclosure.